Research in the linking of organic building units through strong bonds has yielded microcrystalline materials of covalent organic frameworks based entirely on strong covalent linkages between building units. These covalent organic frameworks (COFs) represent an emerging family of crystalline porous materials, with a well-defined and predictable network of molecular building blocks. These materials have potential applications in the field of gas storage, opto-electronics, catalysis and sensing because of their crystalline and periodic structure with high surface area and thermal stability. COFs are usually synthesized as microcrystalline powders and their long range growth is limited in the nano/micro domain due to the internal defects. COF crystallites have been reported to have adopted various shapes such as belts, fibres, sheets, cubes, rectangular prisms etc. However, the mechanisms of the formation and self-assembly of COF crystallites are still poorly understood. The shape of a crystallite is an extremely important factor in molecular absorption and catalysis. In this regard, hollow spherical structures are considered to be an important morphology because of their potential application in catalysis, drug delivery, as molecular sensors and in energy storage. Although active research on the design and synthesis of COFs has been ongoing for almost a decade, there has been only one report of a COF with hollow spherical morphology without much structural (due to poor crystallinity) and mechanistic insight of the hollow sphere formation (Microporous organic network hollow spheres by Kang N et al published in J Am Chem Soc. 2013 Dec. 26; 135(51):19115-8). These hollow microporous organic networks (H-MONs) were prepared by a template method using silica spheres. Moreover, the mixed morphology of the sheets and spheres are reported to be found in the reaction medium thus making the isolation of pure hollow spheres extremely difficult and the chemical instability makes this hollow spherical COF inefficient for any practical use.
Chen Long et al have reported a general synthetic strategy for converting the representative typical porous structure of two-dimensional covalent open lattice structures into ordered donor acceptor heterojunctions (Journal of the American Chemical Society (2014), 136(28), 9806-09). This donor-acceptor strategy by Chen Long et al explored both skeletons and pores of COFs for charge separation and photo-energy conversion. Inspite of the synthesis of porous COF's having the mechanism of donor acceptor heterojunctions, the synthesized COF's do not suggest the loading or immobilization of COF's with biomolecules, or other medically important compounds.
Self-templated synthetic methods are considered to be the most cost effective synthetic methods for hollow sphere synthesis, since they do not need any sacrificial templates. Template free methods further avoid problems such as inevitable shell collapse and the contamination of pores during the template removal. Although self templating has been the major recipe for the synthesis of metal oxide and metal sulfide based hollow spheres, however, self templating method has been less explored to synthesise organic and polymer based hollow spheres.
The present inventors have in an earlier patent application, PCT International Publication No. WO2014203283 disclosed a covalent organic framework comprising porphyrin linked with a hydroxyl aromatic compound synthesized by a self templating method, however the end product synthesized does not yield hollow COF's to facilitate loading or immobilization of biologically important molecules.
The application of COFs are still mostly limited to the storage of gas molecules, since most of the COFs synthesized are microporous in nature and their pores are not large enough to hold the bigger molecules like drugs or enzymes. Even though few mesoporous boronic acid based COFs have been reported in the literature, however, their chemical instability prevents the usage of these materials for the storage of drugs and enzymes.
Therefore, there remains a need in the art to provide covalent organic frameworks (COFs) that are chemically stable, hollow, spherical having mesoporous walls with high surface area so as to meet a variety of biomedical and industrial applications.